The present invention relates to a dual-source fluid supply system for a hydraulic circuit and more particularly to a steering system for a vehicle wherein two sources or pumps which deliver fluid under pressure to a steering circuit are driven by different elements of the vehicle to assure a continuous supply of fluid.
The prior art has recognized the need for auxiliary pumps to assure continuity of hydraulic pressure in vehicular steering circuits during emergency situations. A main or primary pump for supplying fluid under pressure to a vehicular steering circuit is commonly driven directly by the vehicle engine or prime mover. Accordingly, depressurization of the steering circuit may occur when the vehicle engine stops operating or when the primary engine-driven pump fails or becomes inoperative. A pressure failure in the steering circuit may have severe consequences, particularly upon large vehicles such as earth moving machinery where the size of the machinery makes manual steering difficult if not impossible.
Accordingly, it has become common, particularly for large machines, to provide a secondary or auxiliary pump for delivering fluid under pressure to the steering circuit in the event that the main pump fails or becomes inoperative for one reason or another. The auxiliary pump may be driven by rotation of a vehicle wheel, for example, through an output element of a vehicle transmission. Such auxiliary pumps are also commonly designed for reversible operation so that the pump will be capable of delivering fluid under pressure to the steering circuit regardless of the direction of vehicle operation.
Typical references disclosing the use of an auxiliary or secondary pump to provide fluid under pressure when a primary pump fails include U.S. Pat. Nos. 3,625,240; 3,407,894; 3,424,262; and 3,613,818, the last noted patent being assigned to the assignee of the present invention. U.S. Pat. No. 3,566,749, also assigned to the assignee of the present invention, further discloses a manually operable, emergency pumping unit for use in the absence of output pressure from a main pump.